This invention relates generally to gas turbine engines and more particularly to bolted joints for joining adjacent rotor disks in such engines.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and the mixture is ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to drive the compressor and provide useful work such as powering an aircraft in flight. The compressor and turbine sections each include a plurality of rotor disks that are joined together for rotation about the engine""s centerline axis. Each rotor disk comprises a central bore region, a disk rim from which a plurality of radially extending blades are supported, and a web joining the bore and rim. The bore and web are typically much more massive than the disk rim to accommodate the stresses to which the disk is subjected.
Rotating disks, particularly those in the high pressure turbine section of an engine, develop high radial thermal gradients during transient operation because of exposure of the disk rim to hot gases. In this case, the rim of the disk has a quick thermal response (i.e., temperature increase) while the web and bore react more slowly due to their high relative mass and their lower temperature environment. The thermal gradient creates large tangential and radial stresses in the web and bore of the disk that are magnified by any stress concentrations such as holes, fillets and the like.
A significant challenge in disk design is to connect multiple disks together without developing high stresses. One method of connection is through the use of bolted joints connecting adjacent disks. Often, at least one of the disks must be bolted through the disk web because of space limitations. In such instances, the bolt holes are located in regions of high thermal gradient and produce high concentrated stresses. This limits the allowable time of operation of the rotor hardware.
One approach to reducing bolt hole stress is to pass relatively hot secondary flow path air (such as compressor discharge air) through each bolt hole to heat the disk from inside the bolt hole. In doing so, the temperature distribution in the area of the bolt hole is made more uniform and the stress is dramatically reduced. While there is a significant benefit to hardware life with this approach, there are also drawbacks. One primary drawback of delivering air through the bolt holes for heating the disk is that during engine transients such as acceleration, the bolts used to clamp the joint together heat up extremely quickly relative to the disk. This thermal mismatch causes the bolts to outgrow the disk in the direction parallel to their axes. This differential axial growth results in a loss of clamp load in the bolts during operation and can potentially pose a low cycle fatigue problem in the bolts. One solution to clamp load loss is to simply increase the clamp load that is originally provided at assembly. However, in many instances the axial load is originally established with respect to the yield strength of the bolt material so that it is not possible to increase assembly clamp load without risk yielding the bolt or aggravating a low cycle fatigue problem.
Accordingly, there is a need for a bolted joint in which bolt hole stresses are reduced while minimizing the problems of clamp load loss and low cycle fatigue.
The above-mentioned need is met by the present invention, which provides a bolted joint for connecting first and second rotor disks in a gas turbine engine. The bolted joint includes a bolt hole formed in the first rotor disk and a tube disposed in the bolt hole such that a channel is defined between the tube and the bolt hole. A bolt is disposed in the tube such that a gap is defined between the bolt and the tube. The gap thermally insulates the bolt from hot fluid in the channel. A first passage provides fluid communication between the channel and a forward cavity, and a second passage provides fluid communication between the channel and an aft cavity. Hot fluid passing through the channel reduces thermal gradients in the first rotor disk. The tube thermally shields the bolt from the hot fluid to minimize differential thermal growth.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.